Fabrication of thin film products such as microelectronic integrated circuits is enhanced by periodic measurements of key characteristics of the product during the fabrication process, enabling on-going process adjustments to enhance quality and yield. Characteristics of interest include film thickness, feature spacing, feature size or critical dimension at or around a specific location or a specific circuit element. Measurements of such characteristics are best made by analyzing the wavelength spectrum of light reflected from the feature or location of interest on the workpiece or wafer. How to infer a measurement of such characteristics from the wavelength spectrum is known, and may use a conventional technique such as rigorously coupled wave analysis, for example. Many measurements may be desired during the processing of each individual wafer, so that the time required to perform each measurement reduces productivity. Each measurement typically requires a significant amount of time, during which the wafer cannot be processed and instead is held in a metrology tool. Moreover, the wafer and metrology optics must be carefully positioned to ensure that the measurement is taken in a homogeneous region of the integrated circuit, within which the characteristic to be measured (e.g., thin film thickness, step height, etc.) is the same across the entire region. There is a need to perform such measurements without detracting from productivity.